Electric motor with field-current-generated magnetic-field brake

ABSTRACT

An electric motor with an electrodynamic braking means, where the same power controller is used in both motor operation and in braking. Therefore, the field current with the separately excited electric motor can be controlled or regulated by the power controller. It is also possible to preset a certain braking time. A measure of the braking current is obtained by sensing the voltage at a braking resistor, so the braking current can also be regulated on the basis of the field current.

FIELD OF INVENTION

The present invention relates to an electric motor with anelectrodynamic brake and with a controller that controls the motorcurrent or motor speed by way of a power controller unit.

BACKGROUND INFORMATION

German Patent No. 3/539/841 C2 has already disclosed a braking means fora series motor, where the polarity of the field coil is reversed in thebraking mode and the field coil is connected in series with the armatureacross a solid-state switching device. In motor operation, however, thepower semiconductor device has practically no effect because it forms aseparate circuit from the motor. If this is to be a variable speedmotor, another power controller unit would be necessary for thecontroller. The circuitry required for braking and motor operation ofthe electric motor is relatively complicated because of the two powercontrollers that must be used.

SUMMARY OF THE INVENTION

The electric motor according to an embodiment of the present inventionhas the advantage that the same power controller can be used in bothmotor operation and in the braking mode. It is especially advantageousthat this double use of the power controller not only reduces thecircuitry but also makes it possible to eliminate an additional heatsink. As a rule, a heat sink takes up a lot of space and therefore isnot desirable.

It is also advantageous for the power controller to control the fieldcurrent that creates the magnetic field in braking. The braking currentcan be controlled to advantage by the control input of the powercontroller. This is preferably accomplished by designing the controller5 as a phase controller, which is provided anyway for the purpose ofspeed control in motor operation.

Due to the controller that is provided for the braking current, thebraking time for braking the motor, which may be connected to a cuttingtool or a saw, can be preset. Especially with circular saws, hedgeclippers and lawnmowers, it is desirable to brake the running tool veryquickly in order to minimize the risk of injury to the operator.

It is also advantageous for the controller to have devices for measuringthe braking current for example, a voltmeter to measure the voltage, oran ampmeter to measure the current as is known in the art. An example ofsuch a device is shown schematically as 16 in FIG. 4. With the help ofthese devices, the field current can be kept constant over the entirebraking operation or adapted to a given curve to keeping the brakingcurrent constant, preferably over the entire braking operation, isdesirable in order to avoid high current peaks when initiating thebraking operation. These current peaks cause extreme brush sparking,which in turn causes excessive and irregular wear on the carbon brushes.This results in relatively gentle braking of the electric motor or handtool.

It is also advantageous for a first change-over switch to be used todirect the motor current through the power controller in motor operationor to reverse the field coil or parts thereof when braking, so the fieldcurrent can generate the braking magnetic field.

In addition, a current limiting resistor is also provided in the fieldcircuit (2,6,7,3,4) to avoid overloading the field coils. Likewise, abraking resistor can also be provided in the armature circuit for thebraking operation. This permits separate adjustment of both the fieldcurrent and the braking current.

Another application of the electric motor according to the presentinvention is in an electric hand tool, where the tool should be brakedas quickly as possible to prevent the danger of injury. This isdesirable with a saw, a grinder, a lawnmower, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block schematic of a first embodiment according tothe present invention.

FIG. 2 illustrates a second embodiment according to the presentinvention.

FIG. 3 schematically illustrates a device powered by an electric motorwhich may be, for example, a hand tool, a grinder, a saw, a lawnmower.

FIG. 4 illustrates a block schematic of an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a first electric motor with an armature 1, a first fieldcoil 2 and a second field coil 3 connected in series. A firstchange-over switch 13 is provided between the first field coil 2 andarmature 1, and contacts 6, 8 are closed in motor operation. The secondfield coil 3 is connected to a line terminal N via a power controller 4,preferably a triac, a bidirectional triode thyristor or a thyristor. Thefirst field coil 2 is connected to a second line terminal P. The controlinput of power controller 4 is connected to a controller 5, which isalso connected to line conductor N. Controller 5 has additional inputsfor the actual speed n_(I) measured and the set speed n_(S) . Inaddition, controller 5 is connected to a second change-over switch 14,which detects the voltage on armature 1 across closed second contacts 9,11 in motor operation. When braking, first contact 9, 10 is closed andthus a braking resistor 15 is connected in parallel with armature 1. Itmay also be advantageous to use a diode that rectifies the brakingcurrent as braking resistor 15, because the diode will allow only ahalf-wave to pass through. On the other hand, an ohmic resistor 15 wouldhave the advantage that it would act as a fuse if controller 4 (triac)is short-circuited.

This design may be based on the field current and the braking current.Since second contact 9, 11 of the second change-over switch 14 is open,the control input of controller 5 does not receive a signal. Therefore,controller 5 can recognize whether motor operation or braking isoccurring.

If the actual speed is not detected directly, the armature voltage inmotor operation can also be used as a control parameter here (contact9/11 closed).

The third change-over contact 6, 7 of the first change-over switch 13 isalso connected in parallel with armature 1 via a current limitingresistor 12. Current limiting resistor 12 may have a certain value,preferably a value of 0 ohm.

Controller 5 is designed as a phase controller and is described inGerman Patent No. 3 739 623, for example. However, any other suitablecontroller 5 may also be used as power controller 4.

The operation of this circuit is described below. In motor operation,fourth contact 6, 8 of first change-over switch 13 is closed and secondcontact 9, 11 of the second change-over switch 14 is also closed. Thenline voltage U is applied to the electric motor via terminal P, N. Thecurrent then flows back over the second line terminal N through firstfield coil 2, armature 1, second field coil 3 and power controller 4.Then the speed preset by a control element via input ns, for example,can be maintained by controller 5. The actual speed is picked up by atachometer of a motor shaft, for example, and sent to input n, ofcontroller 5 so that controller can compare the set speed with theactual speed. This normal motor operation is known per se. In addition,controller 5 picks up the armature voltage across closed second contact9, 11 of the second change-over switch 14 and it thus receivesinformation regarding motor operation or braking.

The motor is excited separately from the line when braking. Whenswitching to braking mode, third contact 6, 7 (dotted line) of the firstchange-over switch 13 is closed, or first contact 9, 10 (dotted line) ofthe second change-over switch 14 is closed. The field current then flowsfrom line terminal P through first field coil 2, optionally currentlimiting resistor 12 and the second field coil 3 as well as powercontroller 4. The magnetic field for creating the braking torque isgenerated with the field current. In addition, the braking current,which is determined essentially by braking resistor 15, flows througharmature 1 and braking resistor 15 as well as first contact 9, 10. Thecontroller detects via open second contact 9, 11 the fact that brakingmode has been initiated, and it preselects the field current to achievea certain braking torque. In braking, the field current is a pulsatingd.c. current, where every second half-wave is preferably triggered by amicrocomputer. The value for the field current, its presetcharacteristic, where the field current can be varied with time, or itcan also be kept constant, is stored in a memory (not shown). In thesimplest case, a comparator may also be used to select a default valuefor the field current. A timing element (not shown) such as a monoflopis also provided for setting a maximum braking time. The braking time isselected so the electric motor will stop safely. Accordingly, themaximum field current or a corresponding voltage that can be detectedwith a tachometer (not shown) can be preset for the braking current fora certain period of time. After the preset time has elapsed, the currentflow in field coil 2, 3 is interrupted, so the braking action subsidesbecause of the lack of current.

The duration of the braking operation can also be detected by means ofthe tachometer and thus the duration of the braking current can becontrolled.

In another embodiment of the present invention, the voltage, which is ameasure of the braking current, can be measured across braking resistor15. Therefore the braking current can be regulated especially easilywith the help of the field current. It is especially easy to measure thebraking current when the field coils are connected asymmetrically, inother words, when control element 4 is wired between armature 1 and afield coil 2, 3 according to FIG. 2.

In another embodiment of the present invention, the electric motor isused to power an electric device. This embodiment is shown schematicallyin FIG. 3. The electric device is shown at 20 and may be, for example, ahand tool, a grinder, a saw, or a lawnmower.

I claim:
 1. An electric motor having an electrodynamic brake and beingcoupled to a line terminal, the electric motor comprising:a firstchange-over switch; an armature coupled to the first change-over switch;a power controller coupled to the armature and having a first input anda second input, wherein signals for controlling the power controller aresupplied to the power controller via the second input, and wherein thepower controller regulates one of a motor current and a motor voltageduring a motor operation mode and a motor braking mode; a controllerhaving a third input and a fourth input, wherein signals for controllingthe controller are supplied to the controller via the fourth input, andwherein the controller is coupled in series with the power controllervia the second input; a second change-over switch having a first contactand a second contact, the second change-over switch being coupled inparallel with the armature; and wherein the first contact of the secondchange-over switch is opened during the motor operation mode and closedduring the motor braking mode and the second contact of the secondchange-over switch is closed during the motor operation mode and openduring the motor braking mode, and wherein the second contact of thesecond change-over switch couples the fourth input to the line terminal.2. The electric motor as recited in claim 1, wherein the controllerincludes a phase controller for regulating a field current.
 3. Theelectric motor as recited in claim 2, wherein the field current isregulated during the motor braking mode via the control input of thepower controller.
 4. The electric motor as recited in claim 3, whereinthe controller regulates a braking current.
 5. The electric motoraccording to claim 4, wherein the controller regulates the brakingcurrent for a predetermined maximum braking time.
 6. The electric motoras recited in claim 4, wherein the controller includes means formeasuring the braking current.
 7. The electric motor as recited in claim6, wherein after the controller measures the braking current, thecontroller maintains the braking current as constant.
 8. The electricmotor as recited in claim 7, wherein the controller includes means forsetting the field current in order to maintain the braking currentconstant.
 9. The electric motor as recited in claim 1, furthercomprising a field coil coupled to the first change-over switch, whereinthe first change-over switch includes a third contact and a fourthcontact, wherein the third contact is coupled in parallel to thearmature, and the fourth contact is coupled to the armature.
 10. Theelectric motor as recited in claim 9, wherein during motor braking mode,the first change-over switch electrically separates the field coil fromthe armature and connects the field coil in series with a field circuitincluding the power controller, the field coil and the third contact.11. The electric motor as recited in claim 10, wherein the field circuitfurther includes a current limiting resistor.
 12. The electric motor asrecited in claim 9, wherein an armature circuit including the armatureand the second contact further includes a braking resistor.
 13. Theelectric motor as recited in claim 1, wherein the electric motor is usedin an electric hand tool.
 14. The electric motor as recited in claim 13,wherein the electric hand tool is a saw.
 15. The electric motor asrecited in claim 13, wherein the electric hand tool is a grinder. 16.The electric motor as recited in claim 13, wherein the electric handtool is a lawnmower.
 17. An electric motor having an electrodynamicbrake, the electric motor comprising:a first change-over switch; anarmature coupled to the first change-over switch; a power controllercoupled to the armature and having a first input and a second input,wherein signals for controlling the power controller are supplied to thepower controller via the second input, and wherein the power controllerregulates one of a motor current and a motor voltage during a motoroperation mode and a motor braking mode; a controller having a thirdinput and a fourth input, wherein signals for controlling the controllerbeing supplied to the controller via the fourth input, and wherein thecontroller is coupled in series with the power controller via the secondinput; a second change-over switch having a first contact and a secondcontact, the second change-over switch being coupled in parallel withthe armature; and wherein the first contact of the second change-overswitch is opened during the motor operation mode and closed during themotor braking mode.